You need to hire someone to design your Web site. What should you look for before signing on the dotted line? Let's look at a few different types of consultants.
In 4 case studies, the median usability improvement was 165% from the first to the last iteration, and the median improvement per iteration was 38%. Iterating through at least 3 versions of a UI design is recommended, since some usability metrics may decrease in some versions if a redesign has focused on improving other parameters.
User interface standards can be hard to use for developers. In a laboratory experiment, 26 students achieved only 71% compliance with a two page standard; many violations were due to influence from previous experience with non-standard systems. In a study of a real company's standard, developers were only able to find 4 of 12 deviations in a sample system, and three real products broke between 32% and 55% of the mandatory rules in the standard. Designers were found to rely heavily on the examples in the standard and their experience with other user interfaces.
Thovtrup, H., and Nielsen, J. (1991). Assessing the usability of a user interface standard. Proc. ACM CHI'91 Conf. Human Factors in Computing Systems (New Orleans, LA, 28 April-2 May), 335-341.